Portable telephones currently use batteries as their main power source. For example, cellular telephones use an attached or internal portable battery, and transportable telephones, sometimes called bag-phones, use a portable battery. Adapters, such as battery eliminator adapters, hands-free adapters, or mobile transceiver adapters, can be connected to a vehicle cigarette lighter to provide an external power source for augmenting the main battery. Switching from the main battery to the external power source and vice versa interrupts the power supply to the telephone and may cause termination of a cellular telephone call in process under certain conditions. For example, Global System for Mobile Communications (GSM) applications incorporate a burst mode that pulls 1.6 A or more during a burst. If the cellular telephone power supply is changed during a GSM burst, a telephone call in process may be terminated abruptly due to a failure to complete the power source switch-over during a limited switching period.
The switching period can be lengthened by using a capacitor as a charge storage device. Currently, a large 3300 .mu.F capacitor with dimensions of 48 mm by 12 mm stores a charge during the switch-over. As portable telephones reduce in size, however, incorporating a large capacitor into a telephone is undesirable. Thus, there is a need for a portable telephone that accommodates a power source change during a telephone call without requiring a large capacitor.